Wafer processing method

ABSTRACT

A wafer processing method includes a polyolefin sheet providing step of positioning a wafer in an inside opening of a ring frame and providing a polyolefin sheet on a back side of the wafer and on a back side of the ring frame, a uniting step of heating the polyolefin sheet as applying a pressure to the polyolefin sheet to thereby unite the wafer and the ring frame through the polyolefin sheet by thermocompression bonding, a dividing step of cutting the wafer by using a cutting apparatus to thereby divide the wafer into individual device chips, and a pickup step of cooling the polyolefin sheet, pushing up each device chip through the polyolefin sheet, and then picking up each device chip from the polyolefin sheet.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a wafer processing method for dividinga wafer along a plurality of division lines to obtain a plurality ofindividual device chips, the division lines being formed on the frontside of the wafer to thereby define a plurality of separate regionswhere a plurality of devices are individually formed.

Description of the Related Art

In a fabrication process for device chips to be used in electronicequipment such as mobile phones and personal computers, a plurality ofcrossing division lines (streets) are first set on the front side of awafer formed of a semiconductor, for example, thereby defining aplurality of separate regions on the front side of the wafer. In eachseparate region, a device such as an integrated circuit (IC), alarge-scale integration (LSI), and a light emitting diode (LED) is nextformed. Thereafter, a ring frame having an inside opening is prepared,in which an adhesive tape called a dicing tape is previously attached inits peripheral portion to the ring frame (the back side of the ringframe) so as to close the inside opening of the ring frame. Thereafter,a central portion of the adhesive tape is attached to the back side ofthe wafer such that the wafer is accommodated in the inside opening ofthe ring frame. In this manner, the wafer, the adhesive tape, and thering frame are united together to form a frame unit. Thereafter, thewafer included in this frame unit is processed to be divided along eachdivision line, thereby obtaining a plurality of individual device chipsincluding the respective devices.

For example, a cutting apparatus is used to divide the wafer. Thecutting apparatus includes a chuck table for holding the wafer throughthe adhesive tape and a cutting unit for cutting the wafer. The cuttingunit includes a cutting blade for cutting the wafer and a spindle forrotating the cutting blade. The cutting blade has a central throughhole, and the spindle is fitted in this central through hole of thecutting blade, so that the cutting blade and the spindle are rotated asa unit. An annular abrasive portion is provided around the outercircumference of the cutting blade, so as to cut the wafer. In cuttingthe wafer by using this cutting apparatus, the frame unit is placed onthe chuck table, and the wafer is held through the adhesive tape on theupper surface of the chuck table. In this condition, the spindle isrotated to thereby rotate the cutting blade, and the cutting unit isnext lowered to a predetermined height. Thereafter, the chuck table andthe cutting unit are relatively moved in a direction parallel to theupper surface of the chuck table. Accordingly, the wafer is cut alongeach division line by the cutting blade being rotated, so that the waferis divided.

Thereafter, the frame unit is transferred from the cutting apparatus toanother apparatus for applying ultraviolet light to the adhesive tape tothereby reduce the adhesion of the adhesive tape. Thereafter, eachdevice chip is picked up from the adhesive tape. As a processingapparatus capable of producing the device chips with high efficiency,there is a cutting apparatus capable of continuously performing theoperation for dividing the wafer and the operation for applyingultraviolet light to the adhesive tape (see Japanese Patent No. 3076179,for example). Each device chip picked up from the adhesive tape is nextmounted on a predetermined wiring substrate or the like.

SUMMARY OF THE INVENTION

The adhesive tape includes a base layer and an adhesive layer formed onthe base layer. In cutting the wafer by using the cutting apparatus, thecutting unit is positioned at a predetermined height such that the lowerend of the cutting blade reaches a position lower than the lower surface(the back side) of the wafer, so as to reliably divide the wafer.Accordingly, the adhesive layer of the adhesive tape attached to theback side of the wafer is also cut by the cutting blade at the time thewafer is cut by the cutting blade. As a result, in cutting the wafer,cutting dust due to the wafer is generated, and cutting dust due to theadhesive layer is also generated. In cutting the wafer, a cutting wateris supplied to the wafer and the cutting blade. The cutting dustgenerated in cutting the wafer is taken into the cutting water and thendiffused on the front side of the wafer. However, the cutting dust dueto the adhesive layer is apt to adhere again to the front side of eachdevice. Furthermore, it is not easy to remove this cutting dust adheredto each device in a cleaning step to be performed after cutting thewafer. Accordingly, when the cutting dust due to the adhesive layeradheres to each device formed on the front side of the wafer, therearises a problem such that each device chip may be degraded in quality.

The present invention has been made in view of the above problem, and itis therefore an object of the present invention to provide a waferprocessing method which can prevent the adherence of cutting dust to thefront side of each device in cutting the wafer, thereby suppressing adegradation in quality of each device chip.

In accordance with an aspect of the present invention, there is provideda wafer processing method for dividing a wafer along a plurality ofdivision lines to obtain a plurality of individual device chips, thedivision lines being formed on a front side of the wafer to therebydefine a plurality of separate regions where a plurality of devices areindividually formed. The wafer processing method includes a ring framepreparing step of preparing a ring frame having an inside opening foraccommodating the wafer, a polyolefin sheet providing step ofpositioning the wafer in the inside opening of the ring frame andproviding a polyolefin sheet on a back side of the wafer and on a backside of the ring frame, a uniting step of heating the polyolefin sheetas applying a pressure to the polyolefin sheet after performing thepolyolefin sheet providing step, thereby uniting the wafer and the ringframe through the polyolefin sheet by thermocompression bonding to forma frame unit in a condition where the front side of the wafer and thefront side of the ring frame are exposed, a dividing step of cutting thewafer along each division line by using a cutting apparatus including arotatable cutting blade after performing the uniting step, therebydividing the wafer into the individual device chips, and a pickup stepof cooling the polyolefin sheet in each of the plurality of separateregions corresponding to each device chip, pushing up each device chipthrough the polyolefin sheet, and then picking up each device chip fromthe polyolefin sheet after performing the dividing step.

Preferably, the uniting step includes a step of applying infrared lightto the polyolefin sheet, thereby performing the thermocompressionbonding.

Preferably, the polyolefin sheet is larger in size than the ring frame,and the uniting step includes an additional step of cutting thepolyolefin sheet after heating the polyolefin sheet, thereby removing apart of the polyolefin sheet outside the outer circumference of the ringframe.

Preferably, the pickup step includes a step of expanding the polyolefinsheet to thereby increase a spacing between any adjacent ones of thedevice chips.

Preferably, the polyolefin sheet is formed of a material selected fromthe group consisting of polyethylene, polypropylene, and polystyrene.

In the case that the polyolefin sheet is formed of polyethylene, thepolyolefin sheet is preferably heated in the range of 120° C. to 140° C.in the uniting step. In the case that the polyolefin sheet is formed ofpolypropylene, the polyolefin sheet is preferably heated in the range of160° C. to 180° C. in the uniting step. In the case that the polyolefinsheet is formed of polystyrene, the polyolefin sheet is preferablyheated in the range of 220° C. to 240° C. in the uniting step.

Preferably, the wafer is formed of a material selected from the groupconsisting of silicon, gallium nitride, gallium arsenide, and glass.

In the wafer processing method according to a preferred embodiment ofthe present invention, the wafer and the ring frame are united by usingthe polyolefin sheet having no adhesive layer in place of an adhesivetape having an adhesive layer, thereby forming the frame unit composedof the wafer, the ring frame, and the polyolefin sheet united together.The uniting step of uniting the wafer and the ring frame through thepolyolefin sheet is realized by thermocompression bonding. Afterperforming the uniting step, the wafer is cut to be divided into theindividual device chips by using the cutting blade. Thereafter, in eachof the plurality of separate regions corresponding to each device chipin the polyolefin sheet, the polyolefin sheet is cooled, and each devicechip is pushed up through the polyolefin sheet, whereby each device chipis picked up from the polyolefin sheet. Each device chip picked up isnext mounted on a predetermined wiring substrate or the like. Note that,when the polyolefin sheet is cooled in picking up each device chip, thepolyolefin sheet shrinks and can be easily peeled off from the devicechip, so that it is possible to reduce a load applied to each devicechip.

In cutting the wafer by using the cutting blade, the polyolefin sheetprovided below the wafer is also cut by the cutting blade. That is, thewafer and the polyolefin sheet bonded to the back side of the wafer arecut together by the cutting blade in a condition where the front side ofthe wafer is oriented upward. Accordingly, cutting dust due to thepolyolefin sheet is generated. This cutting dust is taken into cuttingwater supplied in cutting the wafer and then diffused on the front sideof the wafer. However, since the polyolefin sheet has no adhesive layer,the cutting dust is not attached to the wafer, so that the cutting dustcan be removed more reliably in a cleaning step to be performed later.In this manner, according to the preferred embodiment of the presentinvention, the frame unit can be formed by thermocompression bondingusing the polyolefin sheet having no adhesive layer. Accordingly,cutting dust due to an adhesive layer is not generated in cutting thewafer, so that it is possible to suppress a degradation in quality ofeach device chip due to such cutting dust having adhesion.

Thus, the wafer processing method according to the preferred embodimentof the present invention can exhibit the effect that cutting dust doesnot adhere to the front side of each device in cutting the wafer and adegradation in quality of each device chip divided from the wafer can besuppressed.

The above and other objects, features, and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of a wafer;

FIG. 2 is a schematic perspective view depicting a manner of positioningthe wafer and a ring frame on a holding surface of a chuck table;

FIG. 3 is a schematic perspective view depicting a polyolefin sheetproviding step;

FIG. 4 is a schematic perspective view depicting a uniting step;

FIG. 5 is a schematic perspective view depicting a modification of theuniting step;

FIG. 6 is a schematic perspective view depicting another modification ofthe uniting step;

FIG. 7A is a schematic perspective view depicting a manner of cuttingthe polyolefin sheet after performing the uniting step;

FIG. 7B is a schematic perspective view of a frame unit formed byperforming the step depicted in FIG. 7A;

FIG. 8 is a schematic perspective view depicting a dividing step;

FIG. 9 is a schematic perspective view depicting a manner of loading theframe unit to a pickup apparatus after performing the dividing step;

FIG. 10A is a schematic sectional view depicting a standby conditionwhere the frame unit is fixed to a frame support table set at an initialposition in a pickup step using the pickup apparatus depicted in FIG. 9;and

FIG. 10B is a schematic sectional view depicting a working conditionwhere the frame support table holding the frame unit with the polyolefinsheet is lowered to expand the polyolefin sheet in the pickup step.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will now be describedwith reference to the attached drawings. There will first be described awafer to be processed by a processing method according to this preferredembodiment. FIG. 1 is a schematic perspective view of a wafer 1. Thewafer 1 is a substantially disc-shaped substrate formed of a materialsuch as silicon (Si), silicon carbide (SiC), gallium nitride (GaN), andgallium arsenide (GaAs). The wafer 1 may be formed of any othersemiconductor materials. Further, the wafer 1 may be formed of amaterial such as sapphire, glass, and quartz. The wafer 1 has a frontside 1 a and a back side 1 b. A plurality of crossing division lines 3are formed on the front side 1 a of the wafer 1 to thereby define aplurality of separate regions where a plurality of devices 5 such as ICsand LEDs are respectively formed. The crossing division lines 3 arecomposed of a plurality of parallel division lines 3 extending in afirst direction and a plurality of parallel division lines 3 extendingin a second direction perpendicular to the first direction. In theprocessing method for the wafer 1 according to this preferredembodiment, the wafer 1 is cut along the crossing division lines 3 andthereby divided into a plurality of individual device chips individuallyincluding the plural devices 5.

The wafer 1 is cut by using a cutting apparatus. Prior to loading thewafer 1 into the cutting apparatus, the wafer 1 is united with apolyolefin sheet 9 (see FIG. 3) and a ring frame 7 (see FIG. 3) tothereby form a frame unit. Thus, the wafer 1 is loaded in the form ofsuch a frame unit into the cutting apparatus and then cut into theindividual device chips in the cutting apparatus, in which each devicechip is supported to the polyolefin sheet 9. Thereafter, the polyolefinsheet 9 is expanded to thereby increase the spacing between any adjacentones of the device chips. Thereafter, each device chip is picked up byusing a pickup apparatus. The ring frame 7 is formed of a rigid materialsuch as metal, and it has a circular inside opening 7 a having adiameter larger than that of the wafer 1. The outside shape of the ringframe 7 is substantially circular. The ring frame 7 has a front side 7 band a back side 7 c. In forming the frame unit, the wafer 1 isaccommodated in the inside opening 7 a of the ring frame 7 andpositioned in such a manner that the center of the wafer 1 substantiallycoincides with the center of the inside opening 7 a.

The polyolefin sheet 9 is a flexible (expandable) resin sheet, and ithas a flat front side and a flat back side. The polyolefin sheet 9 is acircular sheet having a diameter larger than the outer diameter of thering frame 7. The polyolefin sheet 9 has no adhesive layer. Thepolyolefin sheet 9 is a sheet of a polymer (polyolefin) synthesized bypolymerizing alkene as a monomer. Examples of the polyolefin sheet 9include a polyethylene sheet, polypropylene sheet, and polystyrenesheet. The polyolefin sheet 9 is transparent or translucent to visiblelight. As a modification, the polyolefin sheet 9 may be opaque. Sincethe polyolefin sheet 9 has no adhesive property, it cannot be attachedto the wafer 1 and the ring frame 7 at room temperature. However, thepolyolefin sheet 9 is a thermoplastic sheet, so that, when thepolyolefin sheet 9 is heated to a temperature near its melting pointunder a predetermined pressure in a condition where the polyolefin sheet9 is in contact with the wafer 1 and the ring frame 7, the polyolefinsheet 9 is partially melted and thereby bonded to the wafer 1 and thering frame 7. That is, by applying heat and pressure to the polyolefinsheet 9 in the condition where the polyolefin sheet 9 is in contact withthe wafer 1 and the ring frame 7, the polyolefin sheet 9 can be bondedto the wafer 1 and the ring frame 7. Thusly, in the processing methodfor the wafer 1 according to this preferred embodiment, all of the wafer1, the ring frame 7, and the polyolefin sheet 9 are united bythermocompression bonding as mentioned above, thereby forming the frameunit.

The steps of the processing method for the wafer 1 according to thispreferred embodiment will now be described. Prior to uniting the wafer1, the polyolefin sheet 9, and the ring frame 7, a polyolefin sheetproviding step is performed by using a chuck table 2 having a holdingsurface 2 a depicted in FIG. 2. FIG. 2 is a schematic perspective viewdepicting a manner of positioning the wafer 1 and the ring frame 7 onthe holding surface 2 a of the chuck table 2. That is, the polyolefinsheet providing step is performed on the holding surface 2 a of thechuck table 2 as depicted in FIG. 2. The chuck table 2 has a circularporous member having a diameter larger than the outer diameter of thering frame 7. The porous member constitutes a central upper portion ofthe chuck table 2. The porous member has an upper surface functioning asthe holding surface 2 a of the chuck table 2. A suction passage (notdepicted) is formed in the chuck table 2, in which one end of thesuction passage is connected to the porous member. Further, a vacuumsource 2 b (see FIG. 3) is connected to the other end of the suctionpassage. The suction passage is provided with a selector 2 c (see FIG.3) for switching between an ON condition and an OFF condition. When theON condition is established by the selector 2 c, a vacuum produced bythe vacuum source 2 b is applied to a workpiece placed on the holdingsurface 2 a of the chuck table 2, thereby holding the workpiece on thechuck table 2 under suction.

In the polyolefin sheet providing step, the wafer 1 and the ring frame 7are first placed on the holding surface 2 a of the chuck table 2 asdepicted in FIG. 2. At this time, the front side 1 a of the wafer 1 isoriented downward, and the front side 7 b of the ring frame 7 is alsooriented downward. In this condition, the wafer 1 is positioned in theinside opening 7 a of the ring frame 7. Thereafter, as depicted in FIG.3, the polyolefin sheet 9 is provided on the back side 1 b (uppersurface) of the wafer 1 and on the back side 7 c (upper surface) of thering frame 7. FIG. 3 is a schematic perspective view depicting a mannerof providing the polyolefin sheet 9 on the wafer 1 and the ring frame 7.That is, as depicted in FIG. 3, the polyolefin sheet 9 is provided so asto fully cover the wafer 1 and the ring frame 7. In the polyolefin sheetproviding step, the diameter of the polyolefin sheet 9 is set largerthan the diameter of the holding surface 2 a of the chuck table 2.Unless the diameter of the polyolefin sheet 9 is larger than thediameter of the holding surface 2 a, there may arise a problem suchthat, when the vacuum from the vacuum source 2 b is applied to theholding surface 2 a of the chuck table 2 in a uniting step to beperformed later, the vacuum may leak from any gap between the polyolefinsheet 9 and the holding surface 2 a because the holding surface 2 a isnot fully covered with the polyolefin sheet 9, so that a pressure cannotbe properly applied to the polyolefin sheet 9.

In the processing method for the wafer 1 according to this preferredembodiment, a uniting step is next performed in such a manner that thepolyolefin sheet 9 is heated to unite the wafer 1 and the ring frame 7through the polyolefin sheet 9 by thermocompression bonding. FIG. 4 is aschematic perspective view depicting the uniting step according to thispreferred embodiment. As depicted in FIG. 4, the polyolefin sheet 9transparent or translucent to visible light is provided so as to fullycover the wafer 1, the ring frame 7, and the holding surface 2 a of thechuck table 2, which are all depicted by broken lines in FIG. 4. In theuniting step, the selector 2 c is operated to establish the ON conditionwhere the vacuum source 2 b is in communication with the porous memberof the chuck table 2, i.e., the holding surface 2 a of the chuck table2, so that a vacuum produced by the vacuum source 2 b is applied to thepolyolefin sheet 9 provided on the chuck table 2. Accordingly, thepolyolefin sheet 9 is brought into close contact with the wafer 1 andthe ring frame 7 by the atmospheric pressure applied to the uppersurface of the polyolefin sheet 9.

Thereafter, the polyolefin sheet 9 is heated in a condition where thepolyolefin sheet 9 is sucked by the vacuum source 2 b, therebyperforming thermocompression bonding. In this preferred embodimentdepicted in FIG. 4, for example, the heating of the polyolefin sheet 9is effected by a heat gun 4 provided above the chuck table 2. The heatgun 4 includes heating means such as a heating wire and an air blowingmechanism such as a fan. Accordingly, the heat gun 4 can heat ambientair and blow the heated air. In a condition where the vacuum from thevacuum source 2 b is applied to the polyolefin sheet 9, the heat gun 4is operated to supply hot air 4 a to the upper surface of the polyolefinsheet 9. Accordingly, when the polyolefin sheet 9 is heated to apredetermined temperature, the polyolefin sheet 9 is bonded to the wafer1 and the ring frame 7 by thermocompression bonding.

Another method for heating the polyolefin sheet 9 may be adopted. Forexample, any member heated to a predetermined temperature may be pressedon the polyolefin sheet 9 against the wafer 1 and the ring frame 7. FIG.5 is a schematic perspective view depicting such a modification of theuniting step. As depicted in FIG. 5, the polyolefin sheet 9 transparentor translucent to visible light is provided so as to fully cover thewafer 1, the ring frame 7, and the holding surface 2 a of the chucktable 2, which are all depicted by broken lines in FIG. 5. In thismodification depicted in FIG. 5, a heat roller 6 including a heat sourceis used. More specifically, the vacuum produced by the vacuum source 2 bis first applied to the polyolefin sheet 9, so that the polyolefin sheet9 is brought into close contact with the wafer 1 and the ring frame 7 bythe atmospheric pressure applied to the upper surface of the polyolefinsheet 9.

Thereafter, the heat roller 6 is heated to a predetermined temperature,and next placed on the holding surface 2 a of the chuck table 2 at oneend lying on the outer circumference of the holding surface 2 a asdepicted in FIG. 5. Thereafter, the heat roller 6 is rotated about itsaxis to roll on the chuck table 2 through the polyolefin sheet 9 fromthe above one end to another end diametrically opposite to the above oneend. As a result, the polyolefin sheet 9 is bonded to the wafer 1 andthe ring frame 7 by thermocompression bonding. In the case that a forcefor pressing the polyolefin sheet 9 is applied by the heat roller 6, thethermocompression bonding is effected at a pressure higher thanatmospheric pressure. Preferably, a cylindrical surface of the heatroller 6 is coated with fluororesin. Further, the heat roller 6 may bereplaced by any iron-like pressure member having a flat base plate andcontaining a heat source. In this case, the pressure member is heated toa predetermined temperature to thereby provide a hot plate, which isnext pressed on the polyolefin sheet 9 held on the chuck table 2.

Still another method for heating the polyolefin sheet 9 may be adoptedin the following manner. FIG. 6 is a schematic perspective viewdepicting such another modification of the uniting step. As depicted inFIG. 6, the polyolefin sheet 9 transparent or translucent to visiblelight is provided so as to fully cover the wafer 1, the ring frame 7,and the holding surface 2 a of the chuck table 2, which are all depictedby broken lines in FIG. 6. In this modification depicted in FIG. 6, aninfrared lamp 8 is provided above the chuck table 2 to heat thepolyolefin sheet 9. The infrared lamp 8 can apply infrared light 8 ahaving an absorption wavelength to at least the material of thepolyolefin sheet 9. Also in the modification depicted in FIG. 6, thevacuum produced by the vacuum source 2 b is first applied to thepolyolefin sheet 9, so that the polyolefin sheet 9 is brought into closecontact with the wafer 1 and the ring frame 7 by the atmosphericpressure applied to the upper surface of the polyolefin sheet 9.Thereafter, the infrared lamp 8 is operated to apply the infrared light8 a to the polyolefin sheet 9, thereby heating the polyolefin sheet 9.As a result, the polyolefin sheet 9 is bonded to the wafer 1 and thering frame 7 by thermocompression bonding.

When the polyolefin sheet 9 is heated to a temperature near its meltingpoint by performing any one of the above methods, the polyolefin sheet 9is bonded to the wafer 1 and the ring frame 7 by thermocompressionbonding. After bonding the polyolefin sheet 9, the selector 2 c isoperated to disconnect the communication of the porous member of thechuck table 2 from the vacuum source 2 b. Accordingly, the suctionholding by the chuck table 2 is canceled.

Thereafter, the polyolefin sheet 9 is circularly cut along the outercircumference of the ring frame 7 to remove an unwanted peripheralportion of the polyolefin sheet 9. FIG. 7A is a schematic perspectiveview depicting a manner of cutting the polyolefin sheet 9. As depictedin FIG. 7A, a disc-shaped (annular) cutter 10 is used to cut thepolyolefin sheet 9. The cutter 10 has a central through hole 10 a inwhich a rotating shaft 10 b is fitted. Accordingly, the cutter 10 isrotatable about the axis of the rotating shaft 10 b. First, the cutter10 is positioned above the ring frame 7. At this time, the rotatingshaft 10 b is set so as to extend in the radial direction of the chucktable 2. Thereafter, the cutter 10 is lowered until the outercircumference (cutting edge) of the cutter 10 comes into contact withthe polyolefin sheet 9 placed on the ring frame 7. That is, thepolyolefin sheet 9 is caught between the cutter 10 and the ring frame 7,so that the polyolefin sheet 9 is cut by the cutter 10 to form a cutmark 9 a. Further, the cutter 10 is rolled on the polyolefin sheet 9along a circular line set between the inner circumference of the ringframe 7 (i.e., the periphery of the inside opening 7 a of the ring frame7) and the outer circumference of the ring frame 7, thereby circularlyforming the cut mark 9 a along the above circular line. As a result, apredetermined central portion of the polyolefin sheet 9 is surrounded bythe circular cut mark 9 a. Thereafter, a remaining peripheral portion ofthe polyolefin sheet 9 outside the circular cut mark 9 a is removed.That is, an unwanted peripheral portion of the polyolefin sheet 9including an outermost peripheral portion outside the outercircumference of the ring frame 7 can be removed.

The cutter 10 may be replaced by an ultrasonic cutter for cutting thepolyolefin sheet 9. Further, a vibration source for vibrating the cutter10 at a frequency in an ultrasonic band may be connected to the cutter10. Further, in cutting the polyolefin sheet 9, the polyolefin sheet 9may be cooled to be hardened in order to facilitate the cuttingoperation. By cutting the polyolefin sheet 9 as mentioned above, a frameunit 11 depicted in FIG. 7B is formed, in which the frame unit 11 iscomposed of the wafer 1, the ring frame 7, and the polyolefin sheet 9united together. That is, the wafer 1 and the ring frame 7 are unitedwith each other through the polyolefin sheet 9 to form the frame unit 11as depicted in FIG. 7B. FIG. 7B is a schematic perspective view of theframe unit 11 in a condition where the front side 1 a of the wafer 1 andthe front side 7 b of the ring frame 7 are exposed upward.

In performing the thermocompression bonding as mentioned above, thepolyolefin sheet 9 is heated preferably to a temperature lower than orequal to the melting point of the polyolefin sheet 9. If the heatingtemperature is higher than the melting point of the polyolefin sheet 9,there is a possibility that the polyolefin sheet 9 may be melted to suchan extent that the shape of the polyolefin sheet 9 cannot be maintained.Further, the polyolefin sheet 9 is heated preferably to a temperaturehigher than or equal to the softening point of the polyolefin sheet 9.If the heating temperature is lower than the softening point of thepolyolefin sheet 9, the thermocompression bonding cannot be properlyperformed. Accordingly, the polyolefin sheet 9 is heated preferably to atemperature higher than or equal to the softening point of thepolyolefin sheet 9 and lower than or equal to the melting point of thepolyolefin sheet 9. Further, there is a case that the softening point ofthe polyolefin sheet 9 may be unclear. To cope with such a case, inperforming the thermocompression bonding, the polyolefin sheet 9 isheated preferably to a temperature higher than or equal to a presettemperature and lower than or equal to the melting point of thepolyolefin sheet 9, the preset temperature being lower by 20° C. thanthe melting point of the polyolefin sheet 9.

In the case that the polyolefin sheet 9 is a polyethylene sheet, theheating temperature in the uniting step is preferably set in the rangeof 120° C. to 140° C. Further, in the case that the polyolefin sheet 9is a polypropylene sheet, the heating temperature in the uniting step ispreferably set in the range of 160° C. to 180° C. Further, in the casethat the polyolefin sheet 9 is a polystyrene sheet, the heatingtemperature in the uniting step is preferably set in the range of 220°C. to 240° C.

The heating temperature is defined herein as the temperature of thepolyolefin sheet 9 to be heated in performing the uniting step. As theheat sources included in the heat gun 4, the heat roller 6, and theinfrared lamp 8 mentioned above, some kind of heat source capable ofsetting an output temperature has been put into practical use. However,even when such a heat source is used to heat the polyolefin sheet 9, thetemperature of the polyolefin sheet 9 does not reach the outputtemperature set above in some case. To cope with such a case, the outputtemperature of the heat source may be set to a temperature higher thanthe melting point of the polyolefin sheet 9 in order to heat thepolyolefin sheet 9 to a predetermined temperature.

After performing the uniting step mentioned above, a dividing step isperformed in such a manner that the wafer 1 in the condition of theframe unit 11 is cut by a cutting blade to obtain individual devicechips. The dividing step is performed by using a cutting apparatus 12depicted in FIG. 8 in this preferred embodiment. FIG. 8 is a schematicperspective view depicting the dividing step. As depicted in FIG. 8, thecutting apparatus 12 includes a cutting unit 14 for cutting a workpieceand a chuck table (not depicted) for holding the workpiece. The cuttingunit 14 includes a cutting blade 18 having an annular abrasive portion(cutting edge) for cutting the workpiece and a spindle (not depicted)for supporting the cutting blade 18 so as to rotate the cutting blade18. The cutting blade 18 has a central through hole for mounting thefront end of the spindle. The cutting blade 18 is composed of an annularbase (hub) having the above-mentioned central through hole and anannular abrasive portion provided along the outer circumference of theannular base. The spindle is rotatably supported in a spindle housing16, and the base end of the spindle is connected to a spindle motor (notdepicted) accommodated in the spindle housing 16. Accordingly, thecutting blade 18 can be rotated by operating the spindle motor. Thechuck table has an upper surface as a holding surface for holding thewafer 1.

When the workpiece is cut by the cutting blade 18, heat is generated bythe friction between the cutting blade 18 and the workpiece. Further,when the workpiece is cut by the cutting blade 18, cutting dust isgenerated from the workpiece. To remove such heat and cutting dust dueto the cutting of the workpiece, cutting water such as pure water issupplied to the cutting blade 18 and the workpiece during the cutting ofthe workpiece. Accordingly, the cutting unit 14 includes a pair ofcutting water nozzles 20 for supplying cutting water to the cuttingblade 18 and the workpiece, in which the pair of cutting water nozzles20 are located so as to face both sides of the cutting blade 18. In FIG.8, only one of the two cutting water nozzles 20 is depicted.

In cutting the wafer 1, the frame unit 11 is placed on the chuck tablein the condition where the front side 1 a of the wafer 1 is exposedupward. Accordingly, the wafer 1 is held through the polyolefin sheet 9on the chuck table. Thereafter, the chuck table is rotated to make thedivision lines 3 extending in the first direction on the front side 1 aof the wafer 1 parallel to a feeding direction in the cutting apparatus12. Further, the chuck table and the cutting unit 14 are relativelymoved in a direction perpendicular to the feeding direction in ahorizontal plane to thereby position the cutting blade 18 directly abovean extension of a predetermined one of the division lines 3 extending inthe first direction.

Thereafter, the spindle is rotated to thereby rotate the cutting blade18. Thereafter, the cutting unit 14 is lowered to a predeterminedheight, and the chuck table and the cutting unit 14 are relatively movedin the feeding direction parallel to the upper surface of the chucktable. Accordingly, the abrasive portion of the cutting blade 18 beingrotated comes into contact with the wafer 1 to thereby cut the wafer 1along the predetermined division line 3 in the feeding direction. As aresult, a cut mark 3 a (groove) is formed along the predetermineddivision line 3 so as to fully cut the wafer 1 and the polyolefin sheet9. After cutting the wafer 1 and the polyolefin sheet 9 along thepredetermined division line 3, the chuck table and the cutting unit 14are relatively moved in an indexing direction perpendicular to thefeeding direction by the pitch of the division lines 3. Thereafter, theabove cutting operation is similarly performed along the next divisionline 3 adjacent to the above predetermined division line 3. Aftersimilarly performing the cutting operation along all of the otherdivision lines 3 extending in the first direction, the chuck table isrotated 90 degrees about its axis perpendicular to the holding surface,so that the other division lines 3 extending in the second directionperpendicular to the first direction become parallel to the feedingdirection. Thereafter, the above cutting operation is similarlyperformed along all the other division lines 3 extending in the seconddirection. After performing the cutting operation along all the otherdivision lines 3 extending in the second direction, the dividing step isfinished.

The cutting apparatus 12 may include a cleaning unit (not depicted)provided in the vicinity of the cutting unit 14. That is, the wafer 1cut by the cutting unit 14 may be transferred to the cutting unit andthen may be cleaned by the cleaning unit. For example, the cleaning unitincludes a cleaning table having a holding surface for holding the frameunit 11 and a cleaning water nozzle adapted to be horizontally moved inopposite directions above the frame unit 11 held on the holding surfaceof the cleaning table. The cleaning water nozzle functions to supplycleaning water such as pure water to the wafer 1. The cleaning table isrotatable about its axis perpendicular to the holding surface. Inoperation, the cleaning table is rotated about its axis and at the sametime the cleaning water is supplied from the cleaning water nozzle tothe wafer 1. During this supply of the cleaning water, the cleaningwater nozzle is horizontally moved in opposite directions along a pathpassing through the position directly above the center of the holdingsurface of the cleaning table. Accordingly, the entire surface of thefront side 1 a of the wafer 1 can be cleaned by the cleaning water.

By performing the dividing step as mentioned above, the wafer 1 isdivided into the individual device chips, which are still supported tothe polyolefin sheet 9. In cutting the wafer 1, the cutting unit 14 ispositioned at a predetermined height such that the lower end of thecutting blade 18 is lower in level than the back side 1 b of the wafer1, in order to reliably divide the wafer 1. Accordingly, when the wafer1 is cut by the cutting blade 18, the polyolefin sheet 9 bonded to theback side 1 b of the wafer 1 is also cut by the cutting blade 18, sothat cutting dust due to the polyolefin sheet 9 is generated. In thecase that an adhesive tape having an adhesive layer is used in the frameunit 11 in place of the polyolefin sheet 9, cutting dust due to theadhesive layer of the adhesive tape is generated. In this case, thecutting dust is taken into the cleaning water supplied from the cuttingwater nozzles 20, and then diffused on the front side 1 a of the wafer1. The cutting dust due to the adhesive layer is apt to adhere again tothe front side of each device 5. Furthermore, it is not easy to removethe cutting dust adhered to the front side of each device 5 in acleaning step of cleaning the wafer 1 after the dividing step. When thecutting dust due to the adhesive layer adheres to each device 5, therearises a problem such that each device chip divided from the wafer 1 maybe degraded in quality.

In contrast, the processing method for the wafer 1 according to thispreferred embodiment has the following advantage. In this preferredembodiment, the polyolefin sheet 9 having no adhesive layer is used inthe frame unit 11 in place of an adhesive tape having an adhesive layer.Although the cutting dust due to the polyolefin sheet 9 is generated andthen diffused on the front side 1 a of the wafer 1 as being taken intothe cleaning water, this cutting dust is not attached to the wafer 1,but it is reliably removed in the subsequent cleaning step. Accordingly,it is possible to suppress a degradation in quality of each device chipdue to the cutting dust.

After performing the dividing step or the cleaning step, a pickup stepis performed to pick up each device chip from the polyolefin sheet 9.The pickup step is performed by using a pickup apparatus 22 depicted inFIG. 9. FIG. 9 is a schematic perspective view depicting a manner ofloading the frame unit 11 to the pickup apparatus 22. As depicted inFIG. 9, the pickup apparatus 22 includes a cylindrical drum 24 and aframe holding unit 26 having a frame support table 30 provided aroundthe cylindrical drum 24. The cylindrical drum 24 has an inner diameterlarger than the diameter of the wafer 1 and an outer diameter smallerthan the inner diameter of the ring frame 7 (the diameter of the insideopening 7 a). The frame support table 30 of the frame holding unit 26 isan annular table having a circular inside opening larger in diameterthan the drum 24. That is, the frame support table 30 has an innerdiameter larger than the outer diameter of the drum 24. Further, theframe support table 30 has an outer diameter larger than the outerdiameter of the ring frame 7. The inner diameter of the frame supporttable 30 is substantially equal to the inner diameter of the ring frame7. The frame support table 30 has an upper surface as a supportingsurface for supporting the ring frame 7 thereon through the polyolefinsheet 9. Initially, the height of the upper surface of the frame supporttable 30 is set equal to the height of the upper end of the drum 24 (seeFIG. 10A). Further, the upper end portion of the drum 24 is surroundedby the inner circumference of the ring frame 7 in this initial stage.

A plurality of clamps 28 are provided on the outer circumference of theframe support table 30. Each clamp 28 functions to hold the ring frame 7supported on the frame support table 30. That is, when the ring frame 7of the frame unit 11 is placed through the polyolefin sheet 9 on theframe support table 30 and then held by each clamp 28, the frame unit 11can be fixed to the frame support table 30. The frame support table 30is supported by a plurality of rods 32 extending in a verticaldirection. That is, each rod 32 is connected at its upper end to thelower surface of the frame support table 30. An air cylinder 34 forvertically moving each rod 32 is connected to the lower end of each rod32. More specifically, the lower end of each rod 32 is connected to apiston (not depicted) movably accommodated in the air cylinder 34. Eachair cylinder 34 is supported to a disc-shaped base 36. That is, thelower end of each air cylinder 34 is connected to the upper surface ofthe disc-shaped base 36. Accordingly, when each air cylinder 34 isoperated in the initial stage, the frame support table 30 is loweredwith respect to the drum 24 fixed in position.

Further, a pushup mechanism 38 for pushing up each device chip supportedto the polyolefin sheet 9 is provided inside the drum 24. The pushupmechanism 38 has, at an upper end thereof, a cooling section 38 aincluding therein a cooling mechanism such as a Peltier element. Thatis, each device chip is adapted to be pushed up through the polyolefinsheet 9 by the pushup mechanism 38 located below the polyolefin sheet 9.Further, a collet 40 (see FIG. 10B) capable of holding each device chipunder suction is provided above the drum 24. Both the pushup mechanism38 and the collet 40 are movable in a horizontal direction parallel tothe upper surface of the frame support table 30. The collet 40 isconnected through a selector 40 b (see FIG. 10B) to a vacuum source 40 a(see FIG. 10B).

In the pickup step, each air cylinder 34 in the pickup apparatus 22 isfirst operated to adjust the height of the frame support table 30 suchthat the height of the upper end of the drum 24 becomes equal to theheight of the upper surface of the frame support table 30. Thereafter,the frame unit 11 transferred from the cutting apparatus 12 is placed onthe drum 24 and the frame support table 30 in the pickup apparatus 22 ina condition where the front side 1 a of the wafer 1 of the frame unit 11is oriented upward. Thereafter, each clamp 28 is operated to fix thering frame 7 of the frame unit 11 to the upper surface of the framesupport table 30. FIG. 10A is a schematic sectional view depicting astandby condition where the frame unit 11 is fixed to the frame supporttable 30 set at the initial position. At this time, the plural cut marks3 a have already been formed in the wafer 1 in the dividing step, sothat the wafer 1 has already been divided into a plurality of individualdevice chips 1 c (see FIG. 10B).

Thereafter, each air cylinder 34 is operated to lower the frame supporttable 30 of the frame holding unit 26 with respect to the drum 24. As aresult, the polyolefin sheet 9 fixed to the frame holding unit 26 byeach clamp 28 is expanded radially outward as depicted in FIG. 10B. FIG.10B is a schematic sectional view depicting a working condition wherethe frame support table 30 holding the ring frame 7 with the polyolefinsheet 9 is lowered to expand the polyolefin sheet 9. When the polyolefinsheet 9 is expanded radially outward as mentioned above, the spacingbetween any adjacent ones of the device chips 1 c supported to thepolyolefin sheet 9 is increased as depicted in FIG. 10B. Accordingly,the contact between the adjacent device chips 1 c can be suppressed, andeach device chip 1 c can be easily picked up. Thereafter, a target oneof the device chips 1 c is decided, and the pushup mechanism 38 is nextmoved to a position directly below this target device chip 1 c asdepicted in FIG. 10B. Furthermore, the collet 40 is also moved to aposition directly above this target device chip 1 c as depicted in FIG.10B. Thereafter, the cooling section 38 a is operated to decrease atemperature thereof, and the pushup mechanism 38 causes the coolingsection 38 a to come into contact with a region corresponding to thedevice chip 1 c in the polyolefin sheet 9, thereby cooling the region.Further, the pushup mechanism 38 is operated to push up the targetdevice chip 1 c through the polyolefin sheet 9. Further, the selector 40b is operated to make the collet 40 communicate with the vacuum source40 a. As a result, the target device chip 1 c is held under suction bythe collet 40 and thereby picked up from the polyolefin sheet 9. Such apickup operation is similarly performed for all the other device chips 1c. Thereafter, each device chip 1 c picked up is mounted on apredetermined wiring substrate or the like for actual use.

Note that, after the region in the polyolefin sheet 9 is cooled by thecooling section 38 a, the polyolefin sheet 9 shrinks, thereby generatinga large stress on an interface between the polyolefin sheet 9 and eachdevice chip. As a result, it becomes easy to peel off each device chipfrom the polyolefin sheet 9. Accordingly, a load applied to the devicechip in peeling off from the polyolefin sheet 9 is reduced.

In the wafer processing method according to this preferred embodimentmentioned above, the frame unit 11 including the wafer 1 can be formedwithout using an adhesive tape having an adhesive layer. Accordingly, incutting the wafer 1, cutting dust due to the adhesive layer of theadhesive tape is not generated, so that this cutting dust does notadhere to each device chip 1 c. As a result, there is no possibilitythat each device chip 1 c may be degraded in quality.

The present invention is not limited to the above preferred embodiment,but various modifications may be made within the scope of the presentinvention. For example, while the polyolefin sheet 9 is selected from apolyethylene sheet, a polypropylene sheet, and a polystyrene sheet inthe above preferred embodiment, this is merely illustrative. That is,the polyolefin sheet usable in the present invention may be formed ofany other materials (polyolefins) such as a copolymer of propylene andethylene and an olefin elastomer.

The present invention is not limited to the details of the abovedescribed preferred embodiment. The scope of the invention is defined bythe appended claims and all changes and modifications as fall within theequivalence of the scope of the claims are therefore to be embraced bythe invention.

What is claimed is:
 1. A wafer processing method for dividing a waferalong a plurality of division lines to obtain a plurality of individualdevice chips, the division lines being formed on the front side of thewafer to thereby define a plurality of separate regions where aplurality of devices are individually formed, the wafer processingmethod comprising: a ring frame preparing step of preparing a ring framehaving an inside opening for accommodating the wafer; a polyolefin sheetproviding step of positioning the wafer in the inside opening of thering frame and providing a polyolefin sheet on a back side of the waferand on a back side of the ring frame; a uniting step of heating thepolyolefin sheet as applying a pressure to the polyolefin sheet afterperforming the polyolefin sheet providing step, thereby uniting thewafer and the ring frame through the polyolefin sheet bythermocompression bonding to form a frame unit in a condition where thefront side of the wafer and the front side of the ring frame areexposed; a dividing step of cutting the wafer along each division lineby using a cutting apparatus including a rotatable cutting blade afterperforming the uniting step, thereby dividing the wafer into theindividual device chips; and a pickup step of cooling the polyolefinsheet in each of the plurality of separate regions corresponding to eachdevice chip, pushing up each device chip through the polyolefin sheet,then picking up each device chip from the polyolefin sheet afterperforming the dividing step.
 2. The wafer processing method accordingto claim 1, wherein the uniting step includes a step of applyinginfrared light to the polyolefin sheet, thereby performing thethermocompression bonding.
 3. The wafer processing method according toclaim 1, wherein the polyolefin sheet is larger in size than the ringframe, and the uniting step includes an additional step of cutting thepolyolefin sheet after heating the polyolefin sheet, thereby removing apart of the polyolefin sheet outside an outer circumference of the ringframe.
 4. The wafer processing method according to claim 1, wherein thepickup step includes a step of expanding the polyolefin sheet to therebyincrease a spacing between any adjacent ones of the device chips.
 5. Thewafer processing method according to claim 1, wherein the polyolefinsheet is formed of a material selected from the group consisting ofpolyethylene, polypropylene, and polystyrene.
 6. The wafer processingmethod according to claim 5, wherein the polyolefin sheet is formed ofpolyethylene, and the polyolefin sheet is heated in the range of 120° C.to 140° C. in the uniting step.
 7. The wafer processing method accordingto claim 5, wherein the polyolefin sheet is formed of polypropylene, andthe polyolefin sheet is heated in the range of 160° C. to 180° C. in theuniting step.
 8. The wafer processing method according to claim 5,wherein the polyolefin sheet is formed of polystyrene, and thepolyolefin sheet is heated in the range of 220° C. to 240° C. in theuniting step.
 9. The wafer processing method according to claim 1,wherein the wafer is formed of a material selected from the groupconsisting of silicon, gallium nitride, gallium arsenide, and glass.